1. Field of the Invention
The present invention relates to buckling actuators used as, for example, optical switch devices and optical shutters, in which supporting beams can be buckled in order to shift a movable member between two alternative positions.
2. Description of the Related Art
Generally, buckling actuators are used as optical switch devices which perform switching of an optical path by shifting a movable member (for example, see U.S. Pat. No. 6,360,033 which is hereinafter referred to as Patent Document 1; and U.S. Pat. No. 6,303,885 which is hereinafter referred to as Patent Document 2).
Conventional optical switch devices generally include a substrate, a movable member disposed above the substrate and being shiftable in a predetermined shifting direction, a stationary member which is disposed on the substrate and supports the movable member, a plurality of supporting beams each of which is connected between the stationary member and the movable member and can be buckled (is bendable) in the shifting direction of the movable member, and a switching mechanism for switching the position of the movable member with an electrostatic force.
The movable member has a thin rod-like body extending in the shifting direction and is supported by the supporting beams disposed on both sides of the movable member in the width direction. Moreover, one end of the movable member is provided with a mirror for reflecting light. The movable member is capable of being shifted between a first switch position in which the mirror enters an optical path and a second switch position in which the mirror moves away from the optical path. Furthermore, the switching mechanism includes electrodes having a comb-like structure, which are respectively provided on the substrate and the movable member. When an electrostatic force is generated between these electrodes, the movable member is shifted to one of the switch positions so as to switch the optical path.
According to a conventional buckling actuator disclosed in Patent Document 1, when the movable member is positioned at, for example, the first switch position, the supporting beams are maintained in an initial state where the supporting beams form a substantially S-shape. When the movable member is shifted to the second switch position, the supporting beams become buckled so as to form a substantially reverse S-shape with respect to the initial state. Thus, the resilient force (spring force) of the supporting beams allows the movable member to be maintained at the corresponding switch position.
The potential energy of the movable member, which is dependent upon, for example, the spring force of the supporting beams, is zero when the supporting beams are in the initial state, i.e. the first switch position, but reaches a maximum value as the supporting beams are bent in the process of the shifting of the movable member. When the supporting beams become substantially reverse-S-shaped in the second switch position, the spring force is directed in the opposite direction. For this reason, the potential energy of the movable member decreases from the maximum value. Accordingly, the maximum value defines a barrier of potential energy between the first and second switch positions, such that the movable member can be maintained at the corresponding switch position.
On the other hand, according to a conventional buckling actuator disclosed in Patent Document 2, each of the supporting beams is shiftable along a longitudinal direction thereof in view of the fact that the distance between the opposite ends of the supporting beam becomes smaller as the movable member is shifted. In such a case, the supporting beams are connected with, for example, narrow sections of the movable member, which are bendable in the width direction of the movable member. This allows the supporting beams to move in the longitudinal direction of the supporting beams by a required amount.
In the conventional art according to Patent Document 1 and Patent Document 2, it is important that the movable member can be stably shifted between the two switch positions, and can be stably maintained at the corresponding switch position. In order to achieve these results, the difference in potential energy (i.e. a barrier ΔE′ (delta E′) of potential energy illustrated in FIG. 8 as conventional art, which will be described later with respect to preferred embodiments of the present invention) must be large.
However, according to the conventional art, it is difficult to provide a design that allows the barrier ΔE′ of potential energy to be large since the supporting beams are in a completely restrained state. If the barrier ΔE′ is small, the movable member may undesirably be shifted back to its initial position due to counter-reaction, or may be shifted to its initial position due to an external force.
In order to increase the barrier ΔE′ of potential energy to stably drive or maintain the movable member, an angle of each supporting beam must be set to be more obtuse with respect to the perpendicular direction of the movable member in order to provide larger positive and negative regions of the spring force of the supporting beams.
For this reason, the conventional art is problematic in view of the fact that the voltage applied to the comb-like electrodes must be large. Although the electrostatic force is originally applied to the comb-like electrodes only in the longitudinal direction, if the voltage applied is large, the force components of the electrostatic force in a direction perpendicular to the longitudinal direction may undesirably be applied to the movable member due to an imbalanced electrostatic force caused by, for example, inconsistent processing. This may cause the comb-like electrode of the movable member to come into contact with the comb-like electrode of the substrate, and may thus lead to a short circuit between the electrodes. Accordingly, this may result in a malfunction of the actuator.
Furthermore, according to the first embodiment of Patent Document 1 and the conventional art of Patent Document 2, the supporting beams are shiftable in the longitudinal direction thereof, i.e. the width direction of the movable member. This moderates a buckling load applied to each supporting beam in its longitudinal direction, and is thus advantageous in that a force required to shift the movable member to the buckling position of the supporting beams can be reduced. However, since the supporting beams can be moved easily in the width direction of the movable member, this structure may be problematic in that the supporting beams may cause the comb-like electrodes to come into contact with each other, thereby causing a short and malfunction of the device as described above.
Furthermore, according to the conventional art, when the supporting beams are buckled in the process of the shifting of the movable member, the narrow sections of the movable member are bent in the width direction of the movable member so as to relieve the spring force generated by the buckled supporting beams. As a result, when the spring force applied to the movable member weakens in the process of the shifting of the movable member, the maximum value of the potential energy (i.e. the barrier of potential energy) that is dependent upon, for example, the spring force, becomes lower.
For this reason, according to the conventional art of Patent Document 1 and Patent Document 2, the movable member can easily overcome the barrier of potential energy, meaning that the movable member can be easily switched back and forth between the first and second switch positions. This is problematic in that the movable member may automatically be shifted even with, for example, a small external impact, thus leading to improper operation and low reliability of the optical switch device.